Many of the bolded characters in the characterization above are apomorphies of subsets of streptophytes along the lineage leading to the embryophytes, not apomorphies of crown-group embryophytes per se.

All groups below are crown groups, nearly all are extant. Characters mentioned are those of the immediate common ancestor of the group, [] contains explanatory material, () features common in clade, exact status unclear.

Age. Magallón and Castillo (2009) tentatively suggest an age of about 122.7 m.y., Magallón et al. (2015) an age of about 129 m.y., and Z. Wu et al. (2014) an age of around 188 m.y. for this node, Vekemans et al. (2012: MRCA Buxales and Trochodendrales) an age of around (124.1-)122.6(-121.1) m.y. or a little less. The age for stem Trochodendrales in Wikström et al. (2003) is about (140-)135, 123(-118) m.y. - but c.f. sister group [B [T + CE]].

Evolution.Divergence & Distribution. Doyle (2013) is inclined to think that the evolution of 2-merous, wind pollinated flowers can be pegged to this node.

Genes & Genomes. For the paleoAP3 duplication, etc., see Lamb and Irish (2003), S. Kim et al. (2004b, 2005a), Zahn et al. (2005a), and especially Kramer et al. (2006); Kramer and Zimmer (2006) note that although the paleoAP3 gene has been found in Proteales and Sabiales, whether or not it occurs in Buxales and Trochodendrales remains unclear. EuAP3/PI are involved in stamen identity very much as paleoAP3/PI, but the former may be involved in petal development in core eudicots. For the loss of the rps2 mitochondrial gene, see Adams et al. (2002b), and for the duplication of the RPB1 and RPB2 genes, which may have occurred in the immediate ancestor of Trochodendrales, see Oxelman et al. (2004) and Luo et al. (2007). This latter is a complicated problem, since Buxaceae have no duplication in either gene and there has also been widespread loss of both genes. See the Gunnerales page for a genome triplication.

Phylogeny. For discussion on the position of Trochodendrales, see the Ranunculales page.

Note: Possible apomorphies are in bold. However, the actual level at which many of these features, particularly the more cryptic ones, should be assigned
is unclear. This is partly because many characters show considerable homoplasy, in addition, basic information for all too many is very incomplete, frequently coming from taxa well embedded in the clade of interest and so making the position of any putative apomorphy uncertain. Then there is the not-so-trivial issue of how ancestral states are reconstructed (see above).

Age. Estimates for the divergence times of the two genera are (113-)106, 95(-88) m.y. Wikström et al. (2001) or (65-)20, 19(-7) m.y.a. (Bell et al. 2010).

Fossils of Trochodendraceae are known from since the late Cretaceous as Nordenskioldia, more properly Nordenskioeldia, close to Trochodendron (see also Friis et al. 2011). They are widely distributed around the northern hemisphere in the Eocene, and are sometimes at very high latitudes (Crane et al. 1991; Pigg et al. 2001, 2007; Taylor et al. 2009; Harrington et al. 2011). However, the hairpin loop in the seed is not found in at least some fossil Trochodendraceae (Crane et al. 1991, but c.f.
Doweld 1998c); postgenital fusion of the carpels may not occur. Both the extant genera are kown fossil from the Eocene onwards (Friis et al. 2011 and references).

Trochodendraceae are trees that may be recognised by their serrate,
exstipulate leaves and their distinctive flowers and fruits. The flowers have several laterally connate carpels arranged in a whorl; nectary is poduced from the abaxial part of the carpel. The styles become displaced to the base on the abaxial side of the follicular fruits and the seeds are winged. Tetracentron has leaves with palmate venation and long, pendulous, spicate, axillary inflorescences, and the broad base of the leaf completely encloses the axillary bud, while Trochodendron has leaves with pinnate venation and short, erect, terminal inflorescences with pedicellate flowers.

Evolution.Divergence & Distribution. Both Trochodendron and Tetracentron were widely distributed in the northern hemisphere is the Caenozoic (Grímsson et al. 2008: Tetracentron; Friis et al. 2011). However, in some fossils assigned to Trochodendron there are paired auricles or foliaceous stipels at the base of the lamina (Pigg et al. 2007).

In Tetracentron the petiole becomes round towards the base, and there is a marginal
flange that tightly and totally envelops the axillary bud; the base of the
petiole is broad and the scar encircles much of the stem. In Trochodendron, on the other hand, not only is nodal anatomy variable and the leaves on the adult plant have a narrow petiole and often lack axillary buds, in the young plant the leaves are more similar to those of Tetracentron (Bailey & Nast 1945; Nast & Bailey 1955). Although Baranova (1983) described both genera as having laterocytic stomata, Metcalfe and Chalk (1950) described and drew the stomata of Trochodendron as being laterocylic, while the stomata are of Trochodendron are cyclocytic,
according to Carlquist (1982)....

The micropyle of Trochodendron is described as being
endostomal in Johri et al. (1992) and Endress and Igersheim (1999), but it looks bistomal in Takhtajan
(1991). In both genera the adaxial side of the carpel develops greatly as the fruits ripen and so the style becomes basal on the abaxial side of the follicle.

Previous Relationships. The relationships of the two genera included in Trochodendraceae have been somewhat obscure, although their apparent lack of vessels had long typed them as being primitive angiosperms. Endress (1986b) compared the sclereids of Trochodendron and the secretory cells of Tetracentron with the cells secreting ethereal oils in magnoliids, etc., and linked the two genera with Cercidiphyllaceae (Saxifragales) and Eupteleaceae (Ranunculales) in an expanded Trochodendrales (c.f. also Takhtajan 1997 and Thorne 2007 in part). Kai-yu
et al. (1993) suggested that Tetracentron should be put in its own order.

Classification. Including Tetracentraceae in Trochodendraceae was an option in A.P.G. II (2003). The two families do have quite a lot in common, as is clear from the fairly lengthy ordinal description, and since both are monotypic, combination is in order (see A.P.G. III 2009).